The present invention relates to an improved focusing device for an optical apparatus such as a camera or microscope. More particularly, the invention relates to a focus detecting device which, like the human eye, can detect a point of focus from not only the contrast of an object but also its differences in hue.
In a conventional focus detecting device, the image of an object is projected onto a sensor made up of a plurality of light-receiving elements. The photoelectric conversion outputs of the light-receiving elements are processed according to a predetermined conventional algorithm such as may be employed for contrast detecting or correlation detecting to thus determine whether or not the image of the object is properly focused.
In the case where the object is a monochromatic flat surface, it is impossible for the above-described device to determine whether or not the object is in focus because the outputs of the light-receiving elements are equal due to the fact that the intensity distribution of such an object does not vary. Even the human eye cannot focus on such objects.
On the other hand, where the object is a uniformly illuminated flat surface divided by a boundary into two areas of different colors, while the human eye can focus on such an object, the above-described prior art focus detecting device cannot. The reason for this is that the human eye utilizes not only brightness variations but also differences in hue to detect a boundary between areas of different color in focusing on such objects. In the above-described prior art device, a photoelectric conversion output is obtained by integrating the product of the spectral distribution characteristic of incident light and the spectral sensitivity characteristic of a sensor. Accordingly, even if the surface has different colors, the outputs of the sensors for the boundary may be equal depending on the particular combination of colors then present. In this case, as in the case of monochromatic incident light, the device cannot determine whether or not focalization is obtained.
Detection of focus using the contrast detecting method is based on the theory that when an image formed at a given plane is in proper focus, the difference in intensity between light and dark areas, that is, the contrast of the image, is a maximum. This situation is shown graphically in FIG. 1. Such a method can be implemented by detecting the point at which the high frequency component of the spatial frequency distribution of an image reaches a maximum. A conventional method of detecting the high frequency component of an image employs a sensor array composed of a plurality of light-detecting elements which have uniform photoelectric characteristics and are arranged in a line as shown in FIG. 2. The example described in FIG. 2 shows five linearly arranged light-detecting elements set in a plane in parallel with the image plane. The high frequency component is detected from the difference between the outputs of adjacent light-receiving elements.
If the object is a monochromatic edge chart having a dark right half (shaded) and a light left half as shown in FIG. 3 and the image of this chart is projected onto the sensor array of FIG. 2, as depicted in FIG. 4, the outputs of the elements of the sensor array will be as shown in FIG. 5. In FIG. 5, the positions of the light-receiving elements are indicated on the horizontal axis and the amplitude of the photoelectric conversion outputs on the vertical axis. The photoelectric conversion output of each element is the product of the intensity of the incident light and the integral, with respect to wavelength, of the product of the spectral sensitivity characteristic of the element and the spectral distribution characteristic of the incident light. Since all of the elements have the same spectral characteristics, the output distribution depends only on the intensity of the incident light. For the monochromatic edge chart shown in FIG. 3, the outputs of the first, second and third elements corresponding to the light region are large, while the outputs of the fourth and fifth elements corresponding to the dark region are small. The output distribution in the boundary between the light and dark regions is detected as a measure of contrast, the value of which changes as the image is brought into focus, as described above in reference to FIG. 1.
The case will now be considered in which an edge chart made up of different hues a and b as shown in FIG. 6 is employed, and the boundary of the edge chart is positioned on the sensor array as shown in FIG. 4 (boundary indicated by triangular marks). The spectral distribution characteristics of the light beams reflected from the different regions a and b and the spectral sensitivity characteristic of the sensors can be represented by a(.lambda.), b(.lambda.) and s(.lambda.), respectively, as shown in FIG. 7. The radiant intensities of the light beams reflected from the different regions can be represented by A and B, respectively. Therefore, the output P.sub.1 of each of the first, second and third elements is: EQU P.sub.1 =A.multidot..intg.a(.lambda.).multidot.s(.lambda.)d.lambda..
Similarly, the output P.sub.2 of each of the fourth and fifth elements is: EQU P.sub.2 =B.multidot..intg.b(.lambda.).multidot.s(.lambda.)d.lambda..
It is apparent from these formulas that if the intensities A and B differ, then the integrated outputs P.sub.1 and P.sub.2 may nevertheless be equal, depending upon the spectral distribution characteristics a(.lambda.) and b(.lambda.). Likewise, if the spectral distribution characteristics a(.lambda.) and b(.lambda.) differ, the integrated outputs P.sub.1 and P.sub.2 may still be equal, depending on the intensities A and B. In the situation where the photoelectric conversion outputs of all the elements are equal, it is impossible to detect the boundary of the edge chart.
Accordingly, an object of this invention is to provide a focus detecting device which can respond to differences in hue as well as intensity, thus providing a wider operating range than a conventional focus detecting device which utilizes only an integration value derived from a measurement of the incident light energy.